There are many applications in industry in which it is desirable to protect a fluid transducer or switch from excess fluid pressure conditions. One example is in use of a vacuum sensor in a system for charging refrigeration equipment with refrigerant. When refrigeration equipment is evacuated of refrigerant for repair, it is necessary to remove air from the refrigeration equipment before recharging with refrigerant. This is typically accomplished by coupling a vacuum pump to the refrigeration equipment, and operating the vacuum pump until pressure within the refrigeration equipment reaches a selected vacuum threshold, such as 1000 micrometers of mercury. A vacuum sensor is employed for advising the operator of the vacuum level within the refrigeration equipment and/or automatically initiating a recharging cycle.
The vacuum sensor in such refrigeration service equipment applications typically comprises a thermistor-based sensor that responds to changes in thermal conductivity in the fluid surrounding the thermistor to indicate fluid pressure. A significant problem with sensors of this type is accuracy limitations. Furthermore, for thermistor-based sensors of the type described, both refrigerant gas and vacuum have insulating affects, so that there exists a zone of ambiguity between vacuum levels from atmospheric pressure down to some level of vacuum where the refrigerant gas no longer masks the true vacuum. Moreover, thermistor-based sensors of this type do not read accurately if coated with lubricant from the refrigeration equipment. Thermistors are capable of resisting high pressure, but can be damaged by the oil normally carried by refrigerant.
An alternative to thermistor-based sensors are linear output pressure transducers using strain gauge, capacitance or piezoelectric technologies. A refrigerating system may experience operating pressure as high as 300 psi or more. If a linear output pressure sensor is to be capable of direct exposure to the fluid at high pressure, it will be insufficiently accurate at low pressure or vacuum conditions to satisfy the necessary requirements. For example, if a sensor is accurate to 0.1% of full scale and is designed to withstand pressures up to 300 psi, then minimum measured accuracy is 0.3 psi or 15,500 micrometers. This is well above the evacuation pressure of 1000 micrometers recommended by ASHRAE.
U.S. Pat. No. 5,172,562 proposes a solution to the problem of protecting a fluid vacuum sensor from high-pressure conditions, in which the vacuum sensor is disposed between solenoid valves, one of which is responsive to a pressure switch coupled to the system inlet for preventing opening of the valve, and fluid access to the vacuum sensor, when inlet pressure is above a preselected level. This arrangement, although effective for the purpose intended, involves a multiplicity of components that increase equipment cost. Furthermore, the arrangement so disclosed is not well suited for use in conjunction with hand-held equipment, in which size and weight are important factors.
It is therefore a general object of the present invention to provide apparatus for protecting a fluid sensor against excess fluid pressure that is economical to assemble, that is compact and light in weight, and that permits use of highly sensitive vacuum sensors in applications that would otherwise be exposed to high fluid pressure conditions. Another object of the invention is to provide apparatus for protecting a fluid sensor as so described that is readily adapted for use in connection with other types of fluid sensors, such as infrared sensors for determining refrigerant type.